It sounds like you might be addicted to New Relationship Energy (NRE).

Relationships definitely can become a cycle of friendship and passion that gets deeper with time. But you don't leave that up to chance. It's not an accident or simply because you found "the one" that this happens. It takes commitment and it takes effort. Showing up for your partner and doing the actions that show you love them, consistently over time. You have to want to keep the love going. And a few days where the fire isn't as strong as before isn't a sign that things are failing btw.

I've been in a relationship for 6 years and I love my partner even more than I did at the start. Because I know him better. But I don't feel the same obsession about him, because I'm securely attached, and I am no longer anxious about the relationship ending. But the way we keep the spark alive is through making the effort to flirt with each other and express deep affection every day.